The present disclosure relates generally to medical devices and methods. Although specific reference is made to magnetic resonance imaging (MRI), the methods and apparatus described herein can be used with many medical imaging and diagnostic procedures and apparatuses.
Magnetic resonance imaging (MRI) relies on the principles of nuclear magnetic resonance (NMR). In MRI, an object to be imaged is placed in a uniform magnetic field (B0), subjected to a limited-duration magnetic field (B1) perpendicular to B0, and then signals are detected as the “excited” nuclear spins in the object “relax” back to their equilibrium alignment with B0 following the cessation of B1. Through the application of additional magnetic fields (“gradients”) to the imaging process, detected signals can be spatially localized in up to three dimensions.
MRI of living subjects generally makes use of water protons found in tissues. In a typical imaging setup, a subject may then be first placed in a uniform magnetic field (B0), where the individual magnetic moments of the water protons in the subject's various tissues align along the axis of B0 and precess about it at the so-called Larmor frequency. The imaged subject may then be exposed to a limited-duration “excitation” magnetic field (B1, generally created by application of a radio-frequency (RF) “pulse”) perpendicular to B0 and at the Larmor frequency, where the net aligned magnetic moment (the sum of all individual proton moments aligned with B0) at equilibrium, m0, is temporarily rotated, or “tipped” toward the plane corresponding to B1 (the “transverse” plane). This results in the formation of a net moment, mt, in the transverse plane. After cessation of B1, a signal may be recorded from mt as it “relaxes” back to m0. The local magnetic field environment of each tissue affects mt relaxation rates uniquely, resulting in tissue differentiation on images. Moreover, magnetic field gradients are typically employed in order to spatially localize the signals recorded from mt. The excitation/gradient application/signal readout process, a so-called “pulse sequence”, may be performed repetitively in order to achieve appropriate image contrast. The resulting set of received signals may then be processed with reconstruction techniques to produce images useful to the end-user.
Advances in the field of Magnetic Resonance Imaging (MRI), such as gradient hardware, high field systems, optimized receiver coil arrays, fast sequences and sophisticated reconstruction methods, provide the ability to perform rapid MRI imaging. In at least some instances, however, the capabilities of an MRI machine may be limited by memory capacity and processing speed. Improved methods and apparatuses for performing rapid MRI imaging, particularly in a memory and processing power limited MRI machine, are therefore desired.
Time-efficient production of time-optimal gradient waveforms that comply with safety and hardware gradient rate-of-change limitations is generally recognized as an important challenge for real-time MRI. While other methods may adequately calculate time-efficient gradient waveforms that conform to hardware and safety rate-of-change limitations, they may take many minutes to compute, and may render them unusable for real-time imaging. Thus, improved methods and apparatuses for providing more time-efficient gradient waveforms that conform to hardware and safety rate-of-change limitations in MRI machines are desired.